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| Home > Publications database > Crystal and spin structure and their relation to physical properties in some geometrical and spin spiral multiferoics |
| Home > Publications database > Crystal and spin structure and their relation to physical properties in some geometrical and spin spiral multiferoics |
| Book | PreJuSER-22244 |
2012
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-802-0
Please use a persistent id in citations: http://hdl.handle.net/2128/7518
Abstract: The aim of the present work has been to synthesize and to investigate crystal and spin structure in some geometrical and spin spiral multiferroics. Multiferroic materials exhibit two or more ferroic properties such as, ferroelectricity, ferromagnetism and ferroelasticity. These materials are considered as prime candidates for future computer data storage and spintronics. There are several classes of magnetoelectric multiferroics classified based on the origin of multiferroicity. The two types of multiferroic compounds investigated in the present dissertation are, geometrically frustrated systems including; hexagonal DyMnO$_{3}$ (hDMO) and orthorhombic HoCrO$_{3}$ (HCO) and spin spiral systems including wolframite type Mn$_{0.9}$Co$_{0.1}$WO$_{4}$ (MCoW) and Mn$_{0.9}$Cu$_{0.1}$WO$_{4}$ (MCuW). The samples were characterized by macroscopic techniques; specific heat and magnetization as well as microscopic techniques; x-ray diffraction and neutron scattering. Polycrystalline samples of HCO were prepared by solid state reaction method and the phase purity is confirmed by x-ray diffraction measurements. From magnetization measurements we determined magnetic ordering temperature and also an indication of antiferromagnetic exchange interactions is found. In isothermal magnetization measurements a hysteretic behavior was observed indicating the presence of ferromagnetism. Two magnetic field induced transitions is also found from isothermal magnetization measurements. The thermal properties of HCO can be very well described in terms of lattice, hyperfine and crystal field interaction contributions. From the specific heat data the contributions from lattice, hyperfine and crystal field interactions was determined. The hyperfine field at Ho site and the hyperfine splitting energy are deduced. Using a Schottky formula for multiple crystal field levels, five crystal field energies were computed. Magnetic ordering of both Cr and and Ho moments in HCO was determined by neutron powder diffraction (NPD) measurements. The Cr magnetic moments order antiferromagnetically with a small canting angle about crystallographic z-direction while the ordering of Ho moments is a induced type. Ho orders antiferromagnetically within ab-plane with weak ferromagnetic component along $\textit{b}$-direction. The thermal variation of Cr and Ho ordered moments was understood based on molecular field (MF) theory. The obtained molecular field constants reveals that Cr–Cr interactions are stronger compared to Ho–Cr interactions. The lowest crystal field splitting energy obtained from MF theory is in good agreement with the value obtained from specific heat and inelastic neutron scattering (INS) measurements. NPD data also indicated the presence of spin fluctuations which was confirmed by INS measurements. From INS data the correlation length of short range ordering was calculated using Selyakov-Scherrer formula. [...]
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